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Commerce Kickstart is a Drupal distribution and has the exact same support for MongoDB like Drupal proper.
Drupal sites never use MongoDB completely, but instead only offload specific storages (field storage, watchdog, etc).
A bit of googling will give you the appropriate resources for using MongoDb with Drupal (it's outside the scope of this issue queue).

The Drupal Commerce blurb you pasted is just retelling what Drupal itself supports.

I noticed that I can use mongodb together with drupal commerce but not together with commerce kickstart. To use mongodb you have to disable the drupal block modul. Thats ok with commerce but several commerce kickstart modules depence on exacly the drupal block modul. If you disable it an all the related kickstart modules you only have a half commerce kickstart but with mongodb.

Drupal is not verry fast and with commerce kiskstart is even worth so it could be an solution to speed it up with mongodb.

The mongodb module for drupal comes with an replacement for the core drupal block. Before using the mongodb block you have to disable the drupal core block module. Is all fine with commerce if you don't use kickstart...